JP2009-236190A discloses a hybrid operating machine utilizing a regeneration flow rate from an actuator. In this hybrid operating machine, an on-off valve is provided between a boom cylinder as the actuator and a hydraulic motor for regeneration. The on-off valve is kept at a closed position when a control valve for controlling the actuator is returned to a neutral position.
In the case of suddenly stopping the boom cylinder operating at a high speed while a high load is acting thereon, the on-off valve is switched to the closed position at the same time as the control valve is switched to the neutral position, thereby preventing runaway of the boom cylinder and preventing a high torque equal to or higher than absorption capacity of a motor generator from being input from the hydraulic motor to the motor generator. This prevents a high torque equal to or higher than the absorption capacity of the motor generator from acting on the motor generator to cause a failure or runaway of the motor generator.